1. Field of the Invention
The present invention relates to improvements in an electrical discharge apparatus, and more particularly to improvements in an electrical discharge apparatus capable of maintaining for long periods of time the effect of lubricating balls or rollers used in sliding portions of mechanically driven parts, and improvements in an electrical discharge apparatus capable of preventing a decline in the operating efficiency due to a condition such as a machining stop during an abnormality of a lubricant supplying unit for supplying a lubricant to the mechanically driven parts.
2. Description of the Related Art
In an electrical discharge apparatus for machining a workpiece by supplying working electric power to a gap between the workpiece and an electrode provided in a working fluid, it is well-known that unless machining debris produced in the machining gap is eliminated by some method, the insulation between the electrode and workpiece and the repetition of electric discharge cannot be maintained in proper states, and that a state of arc discharge occurs, producing adverse effects such as a decline in the machining efficiency and the deterioration of characteristics of the machined surface.
As a technique used in conjunction with the injection, ejection, and suction of a working fluid to discharge the machining debris from the machining gap, the so-called electrode jumping operation is known in which the electrode is intermittently made to undergo reciprocating motion at a high speed. In the case of a machining configuration for which fluid processing, such as the injection, ejection, and suction of the working fluid to discharge the machining debris, cannot be physically used, the electrode jumping operation is the only method of discharging the machining debris and is generally regarded as one of the machining conditions.
An example of this electrode jumping operation is shown in FIG. 12, in which the ordinate shows the position of the electrode, while the abscissa shows the time. Parameters of the electrode jumping operation include an amount of rise of jump, 201, jump time 202, machining time 203, and a jump speed. For example, in a case where the machining depth is large, by securing a sufficiently large amount of rise of the jump, the discharge of the machining debris from the deep machined hole is made possible. Further, the jump speed affects the jump time (wasteful time) which does not contribute to machining as well as the machining-debris discharging efficiency. Thus, the setting of the parameters of the electrode jumping operation is very important to improve the machining rate by discharging the machining debris with high efficiency.
In addition, to improve the machining efficiency, it is the general practice to repeat the electrode jumping operation at a high speed and a high acceleration. For example, the conditions of the electrode jumping operation in which the amount of rise of the jump is 0.2 mm and the maximum rate of arrival reaches 1,000 to 5,000 mm/min are set values which are commonly used frequently, and the speed and the acceleration are very large. Under such conditions of the electrode jumping operation, the load applied to mechanically driving parts, such as a ball screw and guides, which are generally used in the electrical discharge apparatus is very large, and it is difficult to form and maintain oil films of a lubricant on their sliding surfaces.
Further, in the electrical discharge apparatus in which positional changes during machining are very small and machining is carried out for long periods of time as compared with general machine tools, the wear of sliding surfaces of the mechanically driving parts is sometimes accelerated locally. If the wear of the sliding surfaces continues to be accelerated locally, a decline in the machining accuracy, in particular, constitutes a problem in the electrical discharge apparatus for which working accuracy on the order of microns is required. Therefore, to avoid the occurrence of such wear, it is extremely important to supply the lubricant constantly or intermittently to the mechanically driving parts such as the ball screw and the guides.
If the lubricant fails to be supplied to the mechanically driven parts due to the shortage of the lubricant or a failure of a lubricator, an advance in the wear of the mechanically driven parts is acceleratedly promoted, and the dust of wear produced continues to bite into the sliding surfaces, with the result being that the mechanically driven parts are subjected to further damage.
Furthermore, with the electrical discharge apparatus, since the time of performing unmanned operation is long, and high-accuracy machining is required, it is necessary to provide some protective function or other in preparation for such a situation in which the supply of the lubricant is stopped. Accordingly, a protective function is provided for forcibly terminating a machining program in progress in the event that the supply of the lubricant is stopped during the operation of the electrical discharge apparatus. However, if such a protective function is operated, since the time of performing unmanned operation is long in the electrical discharge apparatus as described above, the decline in the operating efficiency due to the stoppage of the scheduled machining in its course has presented a large problem.